1. Field of the Invention
The present invention relates to a magnetic detection apparatus with a sensor (magnetoresistive element) for detecting the strength of an impressed magnetic field by magnetoelectric conversion, and more particularly, it relates to a magnetic detection apparatus for detecting irregularities of a magnetic movable element when power is turned on and thereafter.
2. Description of the Related Art
In the past, in order to detect irregularities of a magnetic movable element when power is turned on and thereafter, for example, it has been well known to use a magnetic detection apparatus with a sensor in the form of a magnetoelectric conversion element (e.g., magnetoresistive element) (see, for example, a first patent document: Japanese patent application laid-open No. 2004-109113).
Hereinafter, reference will be made to a conventional magnetic detection apparatus as described in the above-mentioned first patent document while referring to the accompanying drawings.
FIG. 6 and FIG. 7 are a perspective view and an enlarged plan view, respectively, that show the conventional magnetic detection apparatus. FIG. 8 and FIG. 9 are circuit configuration diagrams showing processing circuits of the conventional apparatus, wherein FIG. 8 shows a circuit configuration for DC processing the amplitude of a detection signal obtained by a change in the resistance of a magnetoresistive element, and FIG. 9 shows a circuit configuration for AC processing the amplitude of the detection signal obtained by a change in the resistance of the magnetoresistive element. FIG. 10 is a timing chart illustrating the operation waveforms of the detection signal DC processed by the circuit configuration of FIG. 8, and FIG. 11 is a timing chart illustrating the operation waveforms of the detection signal AC processed by the circuit configuration of FIG. 9.
In FIG. 6 and FIG. 7, the conventional magnetic detection apparatus includes a magnet 1 for generating a bias magnetic field and a sensor 2 mounted on an upper surface of the magnet 1. The sensor 2 is composed of an IC chip having a first magnetoresistive element 2a and a second magnetoresistive element 2b integrally formed as segments. For example, the first and second magnetoresistive elements 2a, 2b are arranged in opposition to a magnetic movable element 3 and in a side by side relation in the direction of movement of the magnetic movable element 3 (see arrows).
As shown in FIG. 6 and FIG. 7, by arranging the sensor 2 of the magnetic detection apparatus in the vicinity of and in opposition to the magnetic movable element 3 in a side by side relation in the direction of movement of the magnetic movable element 3, the resistance values of the magnetoresistive elements 2a, 2b are changed in accordance with the change in position or movement of the magnetic movable element 3.
In FIG. 8 and FIG. 9, the first and second magnetoresistive elements 2a, 2b are connected in series with each other between a power supply Vcc and ground to form a bridge circuit 10, whereby a detection signal is output from a junction between the first and second magnetoresistive elements 2a, 2b. 
In the DC processing circuit A shown of in FIG. 8, a detection signal c of the bridge circuit 10 is input to a comparison circuit 11 where it is compared with a comparison level VR to be waveform shaped into a rectangular wave. An output signal e of the comparison circuit 11 is then made into a final output signal i corresponding to the magnetic movable element 3 through transistors 12, 13, which is in turn output from an output terminal Vout.
In addition, in the AC processing circuit shown in FIG. 9, a detection signal c of the bridge circuit 10 is input to the comparison circuit 14 as a voltage signal d through a high-pass filter comprising a capacitor 22 and a resistor 23, so that it is compared with a comparison level VRa to be waveform shaped into a rectangular wave. An output signal g of the comparison circuit 14 is then made into a final output signal j corresponding to the magnetic movable element 3 through transistors 12, 13, which is in turn output from an output terminal Vout.
According to the DC processing circuit of FIG. 8, the detection signal c obtained from the bridge circuit 10 in accordance with the movement of the magnetic movable element 3 is converted into the output signal i, as shown in FIG. 10. That is, the detection signal c is converted into the rectangular wave signal e by the comparison level VR of the comparison circuit 11, and the rectangular wave signal e is made into the final output signal i through the transistors 12, 13.
In addition, according to the AC processing circuit of FIG. 9, the detection signal c is converted into the output signal j, as shown in FIG. 11. That is, the detection signal c is made into the voltage signal d with its phase advanced through the high-pass filter comprising the capacitor 22 and the resistor 23, and the voltage signal d is converted into the rectangular wave signal g by the comparison level VRa of the comparison circuit 14, and the rectangular wave signal g is made into the final output signal j through the transistors 12, 13.
Next, reference will be made to an operation of the conventional apparatus as shown in FIGS. 6 through 11 in relation to a problem thereof while referring to FIG. 12 and FIG. 13. FIG. 12 is a timing chart illustrating operation waveforms in the case of the DC processing circuit of FIG. 8 being used, wherein the operation waveforms at normal or room temperature (R.T) and at high temperature (HOT) are respectively shown in comparison with each other. FIG. 13 is a timing chart illustrating operation waveforms in the case of the AC processing circuit of FIG. 9 being used, wherein the operational waveforms at high speed rotation and at low speed rotation are respectively shown in comparison with each other.
In FIG. 12, a temperature offset is caused by variation in the resistance temperature coefficients of the magnetoresistive elements 2a, 2b or variation in the strength of the magnetic field impressed to the magnetoresistive elements 2a, 2b between a detection signal c at room temperature (R.T) and a detection signal c′ at high temperature (HOT). Here, it is found that a large temperature-induced displacement or shift in the rising and falling points of the rectangular wave signals e, e′ is caused by a temperature offset between the detection signals c, c′, and hence a difference Δi between the temperature characteristics of the final output signals i, i′ becomes large. Similarly, it is also found in FIG. 13 that a large shift is caused in the rising and falling points of the rectangular wave signals g, g′ in accordance with a difference in the number of revolutions per minute of the magnetic movable element 3, thus resulting in an increased difference Δj between the rotation number characteristics of the final output signals i, i′.
Since in the conventional magnetic detection apparatus, only the DC processing circuit of FIG. 8 or the AC processing circuit of FIG. 9 has been used, there arises the following problem. That is, a temperature offset is caused in the detection signals c, c′ due to variation in the magnetoresistive elements 2a, 2b and/or in the strength of magnetic field, so that the difference Δi between the temperature characteristics of the final output signals becomes large, as shown in FIG. 12, or the difference Δj between the temperature characteristics of the final output signals becomes large by the difference in the number of revolutions of the magnetic movable element 3, as shown in FIG. 13.